21.

A coil of resistance 400 is placed in magnetic field. If the magnetic flux Φ (Wb) linked with the coil varies with time t  (sec) as Φ = 50t² +4.
The current in the coil at t= 2 s is

• 0.5 A

• 0.1 A

• 2 A

• 2 A

Two similar coils of radius R are lying concentrically with their planes at right angles to each other. The currents flowing in them are I and 2I, respectively. The resultant magnetic field induction at the centre will be

The potential energy of a particle in a force field is, where A and B are positive constants are r is the distance of a particle from the centre of the field. For stable equilibrium, the distance of a particle is

• B/2A

• 2A/B

• A/B

• A/B

In an electrical circuit R, L, C and AC voltage source are all connected in series when L is removed from the circuit, the phase difference between the voltage and the current in the circuit is π/3. If instead, Cis removed from the circuit, the phase difference is again π/3. The power factor of the circuit is

• 1/2

• 

• 1

• 1

A millivoltmeter of 25 mV range is to be converted into an ammeter of 25 A range. The value (in ohm) of necessary shunt will be

• 0.001

• 0.01

• 1

• 1

A compass needle which is allowed to move in a horizontal plane is taken to a geomagnetic pole. it

• will become rigid showing no movement

• will stay in any position

• will stay in the north-south direction only

• will stay in the north-south direction only

The electric field associated with an electromagnetic wave in vacuum is given by E=i 40 cos (kz-6x10⁸t), where E, z= and t are in volt/m, metre and second respectively. The value of wave vector k is

• 2 m^-1

• 0.5 m^-1

• 6 m^-1

• 6 m^-1

What is the flux through a cube of side a if a point charge of q is a one of its corner?

A ring is made of wire having a resistance R_o = 12Ω. Find the points A and B, as shown in the figure, at which a current carrying conductor should be connected so that the resistance R of the subcircuit between these points is equal to 8/3Ω

If voltage across a bulb rated 220 V -100W drops by 2.5% of its rated value, the percentage of the rated value by which the power would decrease is

• 20%

• 2.5%

• 5%

• 5%